ports and the use of an all glass chromatographic system would help to reduce this problem. Based on an investigation of dilution and cross-linking effects on surface area, pore volume, average pore size, and pore size distribution of the resulting polymer and of the relation of these surface properties to actual column characteristics, broad limits for the practical ranges of initial dilution n-heptane and and cross-linking-approximately 30-95 wt DVB 30 or greater mole of monomer reactants-have been formulated. Although no investigation was attempted on
z
other diluent systems or on the addition of other monomers such as acrylonitrile, methyl methacrylate, etc., to alter the chemical nature of the copolymers, there is no apparent reason why properties closely duplicating any of the polymer bead types could not be produced in support-bonded polymers as well as other entirely new types.
z
RECEIVED for review December 8, 1971. Accepted May 1, 1972.
Electrochemical Cell as a Gas Chromatograph-Mass Spectrometer Interface W. D . Dencker and D. R. Rushneckl Northgate Laboratories, Hamden, Conn. 06514
G . R. Shoemake2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. 91 103
An electrochemical cell employing palladium alloy diffusion electrodes has been developed to remove the hydrogen carrier gas exiting a gas chromatograph. Electrochemical pumping removes greater than 99.9996% of the hydrogen, resulting in mass spectrometer inlet pressures less than Torr. This report gives construction details, electrode activation procedures, and performance characteristics of the cell.
THEUSE OF PALLADIUM ALLOY tubes to remove hydrogen from a carrier gas stream has been described by Lucero and Haley ( I ) and Simmonds et al. (2). In addition, Lucero (3) has given a theoretical treatment of an electrochemical cell employing palladium alloy electrodes for use as a GC/MS interface. This paper reports results of the investigation on which some of the work of Lucero (3) was based. A simplified diagram of the cell is shown in Figure 1. Hydrogen and sample from the gas chromatograph (GC) are fed into a palladium/silver (75:25) tube which serves as the cell anode. As this gas mixture flows through the tube, hydrogen selectively permeates the tube wall and is removed while the sample flows into the mass spectrometer (MS). The hydrogen is transported through the electrolyte following a potential gradient, permeates through the cathode, and is vented or recycled. When the cell is operated in electrochemical balance, the quantity of hydrogen removed at the anode is equal to the quantity of hydrogen generated at the cathode. Although the exact mechanisms of hydrogen transfer are complex and are not well defined, the migration of hydrogen through the Present address, Interface, Inc., Box 297, Fort Collins, Colo. 80521 (Author to whom requests for information should be addressed). * Present address, Texas Engineering & Science Consultants, Houston, Texas 77055. (1) D. P. Lucero and F. C. Haley, J. Gas Cliromatogr., 6 , 477
(1968). (2) P. G. Simmonds, G. R. Shoemake, and J. E. Lovelock, ANAL. CHEM., 42, 881 (1970).
( 3 ) D. P. Lucero, J . Chromntogr. Sci., 9, 105 (1971).
metal is brought about by differences in hydrogen partial pressure on each side of the metal barrier. The use of an electrolyte reduces the effective partial pressure of hydrogen to zero on the electrolyte side of the anode and increases the effective partial pressure of hydrogen to a high value on the electrolyte side of the cathode. Notice that the processes are different for the anode and cathode: on the electrolyte side of the anode there is a hydrogen vacuum whereas on the electrolyte side of the cathode there is a hydrogen excess. The hydrogen “vacuum” on the electrolyte side of the anode is brought about because hydrogen molecules cannot exist at the metal-electrolyte interface. If a hydrogen molecule existed, it would become adsorped on the metal and split into ions. These ions would then combine with hydroxyl ions in the electrolyte to form water. Since hydrogen molecules cannot exist, the effective partial pressure of hydrogen is zero, thus providing a large pressure difference across the tube wall. At the inlet end of the anode, the palladium metal is rich in hydrogen, and the transmission of hydrogen through the metal is controlled by the permeability of the metal. In this case, exchange occurs between hydrogen in the gas phase and adsorped hydrogen. Near the outlet end of the anode, the metal is starved of hydrogen, and transmission is controlled by the frequency of collision of hydrogen molecules with the metal tube walls. In designing the cell, it is important that the outlet end of the anode be kept isolated from all sources of hydrogen so that the metal remains starved of hydrogen. The hydrogen “excess” on the electrolyte side of the cathode is brought about by the decomposition of water into hydroxyl ions and hydrogen. The hydrogen thus generated is free to permeate the cathode or to evolve as gas bubbles from the electrolyte side of the cathode surface. Indeed, when a solid metal counter electrode is substituted for the tube, gas molecules evolve at the electrode surface. If the cathode is not sized and activated properly, pressure can build up within the cell, reducing cell efficiency and presenting the danger of cell rupture. An increase in the permeation rate of hydrogen through the cathode can be achieved by purging the gas phase side of the cathode with oxygen or air, or by pumping. These
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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HYDROGEN MIXTURES IN
RESIDUAL G Y E S OUT
HYDROGEN OUT
Figure 1. Simplified diagram of electrochemical separator
ELECTRICAL CONNECTION
WATER VAPOR FEED
7TEFLON LID THERMOCOUPLE WELL
TEFLONBATH
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HEATER WINDINGS
I
STIRRING
ELECTROLYTE LEVEL
PLATINUM ELECTRODE
nor PLATE
I
Figure 2. Electrolyte purification system
techniques reduce the internal cell pressure also, but eliminate the capability of recycling the hydrogen. From the foregoing discussion it can be seen that the mechanisms of hydrogen transfer are different for the anode and cathode. These differences necessitate different treatment procedures in order to obtain maximum hydrogen transmission through each of the electrodes. CELL DESIGN
The equations necessary to design a cell for a given flow rate range have been published by Lucero (3). These equations show that the important parameters which affect cell design are electrode surface area, alloy composition, cell temperature, and gas conductance of the anode and cathode wall. This particular cell was designed to operate in the region of 0-7 std ml per min for use with capillary gas chromatographic columns, and is based on the gas phase separator already described (2). Materials of Construction. Because of the potentials being applied to the electrodes, and because all parts of the cell are immersed in hot concentrated sodium hydroxide/water solutions, a materials testing program was undertaken. The major problem was that of finding an insulator which would 1754
withstand the hot caustic. The only insulating material tested which was not attacked after two days continuous immersion in 7 0 x sodium hydroxide in water at 200 OC was pure polytetrafluoroethylene, and extensive use of this material was made for cell insulators and seals. These results confirmed those obtained by Clifford et al. (4). In addition to palladium and silver as metallic materials of construction, gold and platinum were also satisfactory. All materials were cleaned before use: the polytetrafluoroethylene parts were cleaned in hot aqua regia, rinsed with distilled water, and baked in air at 200 OC; the metals were cleaned in a suitable acid. rinsed with distilled water, and air dried. Electrolyte Choice and Purification. The procedure used to purify the sodium hydroxide/water electrolyte was essentially that of Clifford et al. ( 4 ) and consisted of immersing platinum electrodes in the electrolyte at 200 "C and applying a potential of 1.2 volts while the electrolyte was being stirred. The electrolyte purification system is shown in Figure 2. After the current through the solution dropped below 1 mA and the solution appeared clear and transparent, the electrodes were removed (with the potential still applied), and the electrolyte was ready for use. In this program, hydroxides of lithium, sodium, and potassium were tested. Palladium alloy anode dissolution occurred when potassium hydroxide was used or when combinations of hydroxides employing potassium hydroxide were used. Analyses of electrolytes employing potassium hydroxide showed large (5 %) concentrations of palladium after operation in the cell for 48 hours at anode potentials in excess of 0.4 V. For this reason, potassium hydroxide was strictly avoided in further studies. Sodium hydroxide was then chosen rather than lithium based on work by Clifford et al. ( 4 ) and because it is used in commercial hydrogen generators employing palladium alloy cathodes (5). Cell Geometry. An exploded view of the cell is shown in Figure 3. The anode is constructed of a 24-in. length of palladium/silver (75 :2 5 ) tubing, 0.006-in. i.d. by 0.012-in. o.d., which is wound on a polytetrafluoroethylene mandrel. The ends of the tubing are pure gold (99.9%) soldered into 0.032411. 0.d. by 0.016-in. i.d. gold/platinum (95 :5) tubes which are in turn pure gold soldered into the end cap. The (4) J. E. Clifford, E. S. Kolic, and C . L. Faust, U S . Tech. Doc. Rep., AMRL-TDR-64-44(1964). (5) J. K. Jacobsen, ANAL.CHEM., 37,319 (1965).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
Figure 3. Exploded view of separator
HYDROGEN OUTLET
end cap is made from palladium/silver (75:25) alloy stock. A palladium silver nut and washer hold the mandrel to the end cap. The cathode is constructed of a palladium/silver (75: 25) tube, 0.04341. 0.d. by 0.035-in. i.d. by 12 in. long. It is plugged at one end with pure gold solder and is pure gold soldered to a piece of gold/platinum tubing at the other end. The gold/platinum tube exits the cell through a specially developed feedthru (described in detail elsewhere). The cell base and end cap are made from gold/silver/platinum (69:25: 6) tubing and sheet stock, respectively. All joints are electron beam welded unless specified otherwise. Affixed to one end of the case is a palladium/silver (75:25) ring which provides strength in the area of the gold O-ring. Stainless steel flanges are used to crush the O-ring to give a hermetic seal for initial testing. After initial tests are complete, the palladium/silver ring and end cap are electron beam welded together. The flanges are then removed and the cell is ready for use. Anode Cleaning and .4ctivation. Many methods of palladium alloy electrode treatment have been tested in this and other laboratories (4, 6-10). The method used for anode cleaning and activation by this laboratory closely parallels that of Clifford et al. (4), and consists of washing the palladium/ silver tubing in benzene, acetone, water, and ethanol, annealing in air at 850-900 "C,then heating in air to approximately 700 OC for at least 16 hours. During these 16 hours of heating, air is sucked through the tubing by use of a vacuum pump (approx. 1 Torr). Under these conditions, organic impurities are removed and the palladium oxidizes. The tubing is then cooled slowly to room temperature. After this cleaning and activation, the tubing is handled only with clean gloves to prevent contamination. Cathode Cleaning and Activation. The method used to activate the cathode combines the work of Clifford et al. ( 4 ) and that of Chodosch and Oswin (9). The tubing is cleaned internally and externally by blasting with glass shot. It is then rinsed with benzene, acetone, water, and ethanol. After drying, the tube is heated conductively in air to 850-900 "C for (6) F. A. Lewis, G. E. Roberts, and A. R. Ubbelohde, Proc. Roy. SOC.London, A220,279 (1953). (7) S . Schuldiner and J. P. Hoare, J. Clwm. Phys., 23, 1551 (1955). (8) P. C. Aben and W. G. Burgers, Trans. Faraday SOC.,58, 1989 (1962). (9) S . M. Chodosch and H. G. Oswin, Reu. Energ. Primaire, 1, (1965). (10) D.'N. Jewett and A. C. Makrides, Trans. Faraday Soc., 61, 932 (1965).
TEFLON L I D
CATHODE ASSEMBLY
L E i I J f J Figure 4. Cathode palladization system
10-20 seconds, then cooled slowly 650-700 "C and permitted to oxidize overnight at this temperature. It is then cooled slowly to room temperature and coiled into the helix shown in Figure 3, clean gloves being used to prevent surface contamination. After coiling, gold alloy tubes are pure silver soldered to each end and the cathode is ready for palladizing. Palladizing consists of charging the palladium alloy with hydrogen, then using this hydrogen to reduce palladium chloride, leaving a surface layer of palladium on the palladium alloy. The system used to palladize the cathode is shown in Figure 4. The cathode is charged by dissociating water using the platinum screen anode, and a dilute NaOH electrolyte. After charging, the assembly is removed from the NaOH, rinsed with distilled water, and immersed in a PdC12solution (2M in HC1). The current through the solution is set at 5 mA, and held at this value for 30 min. The quantity of charge transferred deposits 5 mg/cm2 of palladium on the outside surface of the cathode. The inside surface is palladized by immersing the cathode and platinum screen anode in NaOH electrolyte and passing the PdC12solution (2Min HCl) through the inside of the tube. The current is set at 5 mA for 30 min. This procedure deposits 6 mg/cm* on the inside surface of the cathode. The surface densities of palladium which are deposited on the inside and outside of the cathode have been optimized for maximum transmission of hydrogen through a cathode of this particular size. These values differ slightly from the optimum values obtained by Chodosch and Oswin (9) who used sheet electrodes instead of tubes. It would therefore seem that if a cathode size other than that given is used, the optimum surface density of palladium would need to be redetermined.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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1.0
0.8
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0.4
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I HYDROQEN FLOW RATE INTO SEPARATOR
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Figure 5. Effect of voltage on hydrogen removal I
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Figure 6. Effect of separator temperature on hydrogen removal
After palladizing, the cathode is rinsed extensively with distilled water. A gold alloy tube is removed from one end and this end is squeezed shut and pure gold soldered. The cathode assembly is placed in an air furnace and heated to 700 O C overnight. After cooling, the assembly is leak tested using an MS type leak detector, and inspected for flaws in the surface. If the surface is uniformly grey or grey-black, the cathode will yield maximum transmission efficiency for hydrogen. Cell Operating Temperature. The cell is designed to operate at a continuous temperature of 200 "C. The electrolyte solidifies in the 160-180 "C temperature range. and the cell will not operate with a solid electrolyte. Increasing cell temperature above 200 "C increases the water vapor pressure above the electrolyte, but does not affect cell operation. A 250 O C temperature limit is imposed by the polytetrafluoroethylene insulators. An electrolyte concentration other than that which melts at 160-180 O C can be used if an operating temperature different than that of this particular cell is desired. It must be remembered, however, that the permeation rate of hydrogen through the electrodes is temperature de1756
o
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TEMPERATURE (DEGREESCELSIUS1
-
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-
RESIDUAL HYDRWEN FLOW RATE
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pendent (II), and that low temperature will result in reduced hydrogen transmission. Limitations in Cell Voltage. There is a strict operating limit to the cell voltage. If more than 0.79 volt is applied between the electrodes (either polarity), the electrode which is the anode will begin to dissolve. It is therefore imperative that the cell voltage be limited to 0.79 V as an absolute maximum. An upper limit of 0.70 V is used in this laboratory as a safe limit. The 0.79-V limit also explains why the cell cannot be used as a hydrogen generator. The use of nickel or platinum anodes is recommended for generator operation. Operating Precautions. Because the separator anode transmits 100 of nearly every compound which enters it to the MS, the anode is susceptible to deactivation and overload by many sources which do not detrimentally affect other types of GC/MS interface. Small amounts of column bleed can deactivate the anode if it is exposed over a long time period. Only well conditioned, low-bleed columns should be used with the separator until a working knowledge of its tolerance to the particular liquid phase is known. Sulfur-containing samples and liquid phases should be avoided in general; however, the separator will recover from small quantities of sulfur-containing compounds fairly rapidly. Air leaks into the system must be eliminated in order to reduce the background of water in the MS. In general, the presence of water and nitrogen in the MS represents an air leak upstream of the separator (oxygen is converted to water over the hot palladium surface in the presence of hydrogen), (11) 0. N. Salmon, D. Randall, and E. A. Wilk, Kt~ollsA t . Power Lab. Rep. KAPL-1674-10F2 (1956).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
whereas the presence of water alone represents evolution of water somewhere in the system (column or transfer lines). The hydrogen used as a carrier gas must be pure enough to assure that no contributions to MS background come from this source. Hydrogen generators employing palladium tubes followed by a cold trap to remove water are sources of high purity hydrogen. Otherwise, the hydrogen can be passed through a heated palladium alloy tube upstream of the G C to provide high purity hydrogen. A good test to perform in the startup of a system employing the palladium separator is to connect the hydrogen supply directly t o the separator inlet and test the supply for impurities using the MS. Once the purity of the supply has been ascertained, impurities entering the MS can be pinpointed as coming from other parts of the system. It is also advisable to substitute an empty tube for the column in the GC, heat the system t o a high temperature, and observe the materials entering the MS from the blank system. This additional test assures that the only source of contamination entering the MS will be from the column when it is installed. Reactivation of the Separator. If the separator becomes deactivated during use, it can be reactivated if the poisoning is not too severe. Reactivation in situ can be accomplished by permitting the MS to pump pure oxygen or air through the separator at its operating temperature with no hydrogen present. The impurities which are present on the palladium surface are oxidized as is the palladium itself. Upon reconnection of the hydrogen supply, the oxygen is reduced exposing a fresh palladium surface. This oxidation/reduction process can be repeated several times if desired. No apparent damage to the anode results. If in situ oxidation does not regenerate the anode sufficiently, the separator must then be disassembled and the anode replaced. We have found it useful to have a spare anode assembly on hand to reduce system down time.
L
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TEST SYSTEM
A Barber-Colman Series 5000 Gas Chromatograph was interfaced to a Du Pont Model 490 Mass Spectrometer using the palladium separator. A hydrogen purifier upstream of the G C provided pure hydrogen to the system at a flow rate of 2 std ml per min. Residual hydrogen flow rate into the MS was measured using the Du Pont 490 or an instrument built inhouse. TESTS AND RESULTS
i / 3N3ZN38 l A H 1 3
Effect of Cell Voltage on Residual Hydrogen Flow Rate into the MS. As pointed out earlier in this report, there is a maximum voltage at which the cell can be operated. This maximum is dependent upon the potential at which anode dissolution occurs (0.79 V). The objective of this test was to determine the minimum potential required for complete removal of hydrogen. The result is shown in Figure 5. At a hydrogen flow rate of 2 std ml per min, hydrogen "breaks through" to the MS at potentials below about 0.45 V. Subsequent tests showed that this potential is somewhat dependent on electrode activation, and on hydrogen pressure within the cathode. In no case, however, did the cell require more than 0.5 V for complete removal of hydrogen. Effect of Cell Temperature on Residual Hydrogen Flow Rate into the MS. This effect is shown in Figure 6. For this cell in particular, the freezing point of the electrolyte was approximately 180 "C. As can be seen, hydrogen breaks through to the MS when the electrolyte begins to solidify.
l O H 0 3 l V lAlN3dOSI
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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Hydrogenation of Organics. The effect of the palladium surface on organic compounds has been reported by Simmonds et al. (2), who used a palladium alloy tube suspended in air or oxygen as the GC/MS interface. Tests with the cell described in this report yield similar results; i.e., compounds containing conjugated double bonds or triple bonds are hydrogenated, whereas saturated compounds and compounds containing non-conjugated double bonds are not. Life Test. One of the major concerns about the cell was its ability to withstand repeated heating and cooling, and discontinuous exposure to hydrogen. The electrolyte expands at a high rate in its solid form and the effect of this volume change on the stability of the palladium alloy tubes required investigation. In addition, the palladium alloy tubes themselves expand in the presence of hydrogen and contract when it removed (12). In order to determine if these effects would have an adverse effect on separator operation, a life test was devised. The test was based on a 6-hour heating/cooling cycle as shown in Figure 7. After 2000 hours of continuous testing in this mode of operation, the test was terminated. The cell was disassembled and visually inspected for microscopic cracks or fissures in the palladium alloy tubes. None were found. A sample of the electrolyte was analyzed for traces of palladium and silver. Less than 10 ppm (the limit of detection) of each metal was found. From these tests it could be (12) A. S . Darling, Platinum Metals Rec;., 7,126 (1963).
concluded that the concept, design, and construction of the cell were adequate to ensure reliability. Four more cells of this design have been built and tested since the life test: all have yielded equivalent performance. Chromatogram from Total Ion Current Monitor. Figure 8 illustrates separator performance in the GC/MS operating mode. The column used was a 4-ft by 0.030-in. i.d. stainless tube packed with 3 % Dexsil 300 on Chromosorb W, HP 60/80 mesh followed by a 200-ft by 0.020-in. i.d. stainless tube coated with 5x W/V solution of Dexsil 300/Igepal CO 990 (20 :1). The column temperature was programmed from 50 to 200 "Cat 7.5 OC per minute after a 10-minute isothermal hold at 50 "C. Full scale recorder deflection (indicated by the flat top on the solvent peaks) was equivalent to 15 nanograms per second of material flowing into the MS ion source. Of significance is the MS pressure throughout the analysis. Prior to injection, the ion source pressure was 1.8 X 10-8 Torr, and at the end of the analysis, after the phenylundecane peak had passed, the pressure was approximately 8 x 10-8 Torr. The small peaks produced pressures on the order of Torr whereas the phenylundecane peak generated a presTorr, requiring shut-down of the MS ion source. sure of Several hundred analyses of this type have been made with no change in separator performance. RECEIVED for review February 1, 1972. Accepted May 12, 1972. This work was carried out under Jet Propulsion Laboratory Contract NAS 7-100, sponsored by the National Aeronautics and Space Administration.
Plasma Chromatography of the Mono-Halogenated Benzenes Francis W. Karasek and Oswald S. Tatone Department of Chemistry, Unioersity of Waterloo, Waterloo, Ontario
Using thermal electrons and positive reactant ions from nitrogen gas, both positive and negative plasmagram patterns have been obtained for fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene. The plasmagrams give characteristic qualitative data. Positive plasmagrams show protonated molecular ions containing one and two molecules; the negative plasmagrams, except for the fluorobenzene, show only a strong halogen ion peak, which provides experimental evidence for dissociative electron capture by thermal electrons.
THE TECHNIQUE OF PLASMA CHROMATOGRAPHY (PC) permits characterization and analysis of trace constituents in a gaseous mixture at atmospheric pressure. The instrumentation utilizes a 63Niradioactive beta source to create ions for reaction with trace constituents in a gas to produce characteristic positive and negative ion-molecule complexes. The complexes formed in the reactor section are separated in a coupled ion-drift spectrometer and appear as a recorded plasmagram of separated ion-molecule peaks. Basic features of the method and descriptions of the instrumentation have been presented previously (1-3). Some limited work has been (1) F. W. Karasek, Res./Derelop., 21 (3), 34 (1970). (2) M. J. Cohen and F. W. Karasek, J . Chrornatogr. Sci., 8, 330 (1970). (3) F. W. Karasek, Res.JDecelop, 21 (12), 25 (1970).
1758
reported to demonstrate the qualitative information provided in the positive and negative plasmagrams of such compounds as oxygenated organics ( 4 , 3,polychlorinated biphenyls (6), and the chlorinated dibenzo-p-dioxines (7). Studies by PC are curently being conducted on molecules of biological significance by Griffin, Dzidic, and Carroll (8) and on high molecular weight macro-ions of polymers by Dole (9). The PC technique provides data not only for analytical use but for fundamental studies of ion-molecule reactions and related phenomena such as the mechanism of the gas chromatographic electron capture detector (6) and the stable ionic species observed in mass spectra. The first commercial plasma chromatograph of simplified design was recently installed in the author's laboratory. This BETA-VI instrument has an increased resolution, sensitivity, and stability over that of the prototypes used in previous work and permits (4) F. W. Karasek and M. J. Cohen, J. Chromatogr. Sci., 9, 390
(1971). ( 5 ) F. W. Karasek, W. D. Kilpatrick, and M. J. Cohen, ANAL. CHEM., 43,1441 (1971). (6) F. W. Karasek, ibid.. p 1982. (7) D. I . Carroll, Tech. Rept. No. F-llA, Franklin GNO Corporation, P.O. Box 3250, West Palm Beach, Fla., 33402. (8) G. W. Griffin, Baylor College of Medicine, Houston, Texas 77025, personal communication, 1971. (9) M. C . Dole, Baylor University, Waco, Texas. personal communication, 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1 1 , SEPTEMBER 1972
